The present invention relates to a method of making glass fibers wherein relatively viscous glass is fiberized in a rotary fiberization process at relatively high disc speeds and through relatively large diameter holes to form long, fine diameter glass fibers. Air-laid and wet-laid fibrous mats are made from these glass fibers for various applications, such as but not limited to, battery separators, ASHRAE papers, and HEPA and ULPA filtration papers.
Battery separators, ASHRAE papers and filtration papers such as HEPA and ULPA filtration papers are commonly made from glass fibers. The glass fibers used to form these products normally include fibers made by rotary fiberization processes which attenuate the fibers in a curtain of heated air and hot combustion gases and flame attenuation processes, such as but not limited to, pot and marble flame attenuation processes which attenuate the fibers at higher temperatures in a burner flame.
In a typical rotary fiberization process, one or more rotary fiberizers are located in the upper portion of a collection chamber. Each rotary fiberizer has a rapidly rotating fiberizing disc. A stream of molten glass is introduced into the rapidly rotating disc which contains numerous fine diameter orifices in an annular outer peripheral sidewall. The number and the diameter of the orifices varies. For example, the annular outer peripheral sidewalls typically have from several hundred orifices to tens of thousands of orifices ranging from about 0.010 inches to about 0.025 inches in diameter. As the primary glass fibers exit the orifices in the disc sidewall in a generally horizontal direction, the primary glass fibers are introduced into a generally downward directed hot annular curtain of high velocity heated air and combustion gases surrounding the disc, e.g. an annular curtain of hot gases formed by the combustion gases from a burner and a high velocity air stream emitted from an air ring. The hot, high velocity curtain of heated air and combustion gases attenuates the primary fibers exiting the orifices and forms the primary fibers into staple fibers of lesser diameters (typically having a uni-modal fiber distribution with a mean diameter of 2.8 microns or greater). These fibers pass down through the collection chamber and are normally collected on a moving foraminous collection chain conveyor located in the lower portion of the collection chamber.
In a typical flame attenuation process, such as a pot and marble process or similar processes, continuous primary filaments are pulled from a plurality of orifices located in the bottom of a pot or melter containing molten glass by pull rolls located beneath the pot or melter. The continuous primary filaments are then passed through a filament guide and introduced at generally right angles directly into the high velocity flame of a burner where the continuous primary filaments are attenuated and formed into long, staple glass fibers having very fine diameters. The high velocity combustion gases from the flame then carry the fibers through a forming tube and onto a moving inclined foraminous collection chain conveyor located at the opposite end of the forming tube.
Both of the fiberization processes discussed above, the rotary hot air and combustion gas attenuating processes and the flame attenuation processes, produce fibers over a range of fiber diameters with the fiber diameter distribution within those ranges, when plotted in a graph, forming a generally bell shaped curve. These fiber diameter distributions are uni-modal fiber diameter distributions.
The fibers produced by the above discussed rotary fiberization processes are considerably less expensive to produce than fibers produced by the flame attenuation processes. However, in the past, fibers made by these rotary fiberization processes (having uni-modal fiber diameter distributions with mean diameters that are less than 2.8 microns) become increasingly shorter and more difficult to form using these prior art rotary fiberization processes and become unacceptable for use in many applications, e.g. due in part to their relatively short lengths, high shot content and poor tensile strength. Thus, for products which require long, fine diameter fibers (fibers having a uni-modal fiber diameter distribution with a mean diameter of less than 2.8 microns and more especially 2.5 microns or less, such as but not limited to battery separator media (which require fibers with a mean diameter of about 1.4 microns), ASHRAE papers, and HEPA and ULPA filtration papers, the more costly fibers made by pot and marble flame attenuation and similar flame attenuation processes have been used exclusively or, where the product application permits, in combination with the coarser diameter fibers made by a rotary fiberization process. For example, battery separator media are currently formed from fiber blends having a bi-modal fiber diameter distribution and a mean diameter of about 1.4 microns. In an attempt to provide the required performance characteristics for such products while lowering product costs the mean fiber diameter for such blends (e.g. about 1.4 microns) is typically achieved by including flame attenuated fibers having a uni-modal fiber diameter distribution with a mean diameter of about 0.7 microns and rotary fibers having a different uni-modal fiber diameter distribution with a mean diameter of about 2.8 microns as schematically represented in FIG. 4. A typical fiber blend for such a product thereby has a bi-modal fiber distribution of about 40% by weight fibers having a mean diameter of about 0.7 microns and about 60% by weight fibers having a mean diameter of about 2.8 microns which combined gives the product, as a whole, a mean fiber diameter of about 1.4 microns.
Thus, there has been a need for long, fine diameter glass fibers (fibers having a uni-modal fiber diameter distribution with a mean diameter less than 2.8 microns, preferably less than about 2.5 microns, more preferably less than about 2.0 microns, and most preferably about 1.4 microns) made by a rotary fiberization process, for making air-laid and wet-laid products, such as but not limited to battery separator media, ASHRAE papers, and HEPA and ULPA filtration papers, that have the required physical properties for these applications. Furthermore, there has been a need in the market place for air-laid and wet-laid products, such as those set forth above, that are less expensive than those products currently on the market.
In the preparation and/or subsequent processes and handling of glass fiber products a portion of the glass fibers forming the products are often cut or broken into lengths which may be inhaled. As it is impractical or impossible to remove such fibers from the body, it has become important to create glass compositions which exhibit high degrees of biosolubility, i.e. which are rapidly solubilized in biological fluids.